High vacuum processing units such as vacuum coaters and metallizers routinely use diffusion pumps as a pumping source in the high vacuum range. Typically, a single-seated high vacuum poppet valve is employed to isolate the diffusion pump from the processing unit, thus providing rapid cycling of vacuum in the vacuum process chamber by permitting the diffusion pump to be brought in-line quickly by the simple expediency of having an operator open the poppet valve between the vacuum process chamber and an already operating diffusion pump, thereby obviating the need to wait for the diffusion pump to achieve operating temperature.
To assist in removing the condensable vapors which desorb from the walls of the vacuum process chamber, it is common practice to install a refrigerated surface or "cold trap" and to pump the desorbed vapors therefrom. The cold trap and diffusion pump provide a very effective high vacuum pumping system, especially when conditions of high humidity are present.
Normally, the cold trap is physically located between the single-seated poppet valve and diffusion pump which permits the poppet valve to effectively isolate the cold trap as well as the diffusion pump from the vacuum process chamber whenever the high vacuum system is not needed to pump in the chamber. Such an arrangement however presents several disadvantages:
(1) Placing the trap in front of the diffusion pump creates a partial obstruction of the pump inlet resulting in a reduction in pumping speed to as low as 35% of nominal rated speed.
(2) Defrosting of the cold trap to remove accumulated frozen water vapor therefrom requires the evolved water to pass out through the pump. If the rate of vapor evolution is sufficiently high, a resultant pressure rise in the pump can backstream the pump oil onto the poppet valve for possible eventual migration of the oil into the vacuum process chamber work area.
The present invention involves moving the refrigerated cold trap from a position in front of the inlet of the diffusion pump to another position along their existing common axes to thereby dispose the trap and pump in a parallel relationship. Operating along the existing common axis is a poppet valve provided with dual seats, the upper of which cooperates with a valve disc to form a dome which houses the refrigerated coils or cold trap, and the lower of which coacts with another valve disc cooperating with the diffusion pump. The advantages resulting from the above arrangement will be more fully understood from the drawings and description thereof hereinafter presented.